In optical communication systems, digital to analog conversion (DAC) is an important feature of modern high speed optical transponders that are used for various purposes including the generation of higher order modulation formats (e.g., Quadrature Amplitude Modulation or QAM), digital pulse shaping (Nyquist pulses, root raised cosine pulse shaping), or the pre-distortion of transmit signals to compensate for certain transmission impairments (e.g., fiber nonlinearity) that occur within optical fiber media. At the transmission end of many optical communication systems that use digital modulation techniques, DACs are used as part of the modulation circuitry. Typically, the circuit architecture used for many such digital modulation circuitry comprises a Digital Signal Processor (DSP) that is closely coupled to a DAC, which in turn is coupled to a digital modulator such as an I/Q (In-phase/Quadrature phase) modulator. This DSP/DAC/I/Q Modulator architecture when used in relatively high-speed optical systems presents several problems.
First, the interface between the DSP and the DAC typically handles signals having aggregate speeds on the order of multi-Terabits/second. As a consequence of the speeds involved, the design of such circuits is technically challenging because of the complexity of such circuits and their relatively high power consumption. To address the power consumption needs of these circuits, CMOS (Complementary Metal Oxide Semiconductor) technology is used in implementing them as such technology is known for its low power consumption. Typically, the DSP used is an ASIC (Application Specific Integrated Circuit) that can process multi-Terabits/second aggregate signal streams. Furthermore, in order to apply the proper signal levels to the input of the optical modulators, driver amplifiers with reasonably linear characteristics are required to avoid the distortion that these analog signals would otherwise experience. Such linear drivers are generally expensive and it would be desirable to replace them with saturated non-linear drivers, or even more desirable to omit their usage altogether. Previous attempts to reduce the need for high speed DACs and associated linear electronic driver amplifiers have been made by shifting portions of the DAC functionality into the optical modulator. However, this approach increases the number of modulator circuits needed and requires significantly more difficult bias controls, phase adjustments and optical/electrical path matching when compared to the DSP/DAC/IQ architecture discussed above. Three examples of this approach are discussed below and two of them are shown in FIGS. 1, 2 and 4.
In FIG. 1, a quadruple-nested Mach-Zehnder modulator (MZM) is shown where an optical wave is inputted into an optical waveguide arranged as per the well known tree-like Mach-Zehnder structure; see [1] A. Chiba, et al. 16-level quadrature amplitude modulation by monolithic quad-parallel Mach-Zehnder optical modulator. Electron. Lett., 46:227-228, 2010. [2] H. Yamazaki, et al. 64QAM modulator with a hybrid configuration of silica PLCs and LiNb03 phase modulators for 100-Gb/s applications. Proc. European Conf. on Opt. Commun. (ECOC), paper 2.2.1, 2009. [3] T. Sakamoto, et al. 50-km SMF transmission of 50-Gb/s 16 QAM generated by quad-parallel MZM. Proc. European Conf. on Opt. Commun. (ECOC), paper Tu.1.E.3, 2008.
The inputted optical wave is split into two paths leading to upper and lower arms of the Mach-Zehnder structure. The upper and lower arm each are split into four paths each of which has a modulator positioned proximate the optical waveguide to impart a phase shift to the optical wave by applying voltage to the wave guide at appropriate time instances. The application of a voltage generates an electric or electromagnetic field, which interacts with the optical wave traveling through the waveguide causing the wave to be slightly delayed which delay represents a phase shift. Although not shown, each of the modulators has at least one electrode mounted proximate the wave guide and each modulator applies the modulation voltages through its electrode(s).
FIG. 2 depicts the basic Mach-Zehnder arm structure wherein an optical waveguide is channeled through a substrate of a crystalline material such as Lithium Niobate (LiNbO3) or semiconductor materials such as Gallium arsenide (GaAs) and Indium Phosphide (InP). The phase modulators are positioned proximate a portion of the waveguides and the optical waves from each branch are coupled to or interfere (constructively and/or destructively) with each other at a Y-branch coupler; the light then travels to the output where it typically enters a fiber optic medium for transmission over a network.
FIG. 3 depicts a typical signal constellation for a 16-QAM modulator, which can be implemented with the quad-parallel Mach-Zehnder (MZM) modulator discussed above. FIG. 4 shows another type of optical modulator with DAC functionality commonly referred to as an electro-absorption modulator; see [4] Advanced Optical Modulation Modulation Formats; PROCEEDINGS OF THE IEEE|Vol. 94, No. 5, May 2006, pp. 952-985, Winzer, Peter J.; Essiambre, René-Jean; see also [5] C. R. Doerr et al. Monolithic InP 16-QAM modulator. Proc. Opt. Fiber Commun. Conf. (OFC), paper PDP20, 2008. Here the waveguides are channeled through a PIN (Positive-Intrinsic-Negative) material. PIN describes a process of doping three layers of semiconductor material. As the voltages are applied by the modulators, the bandgap of the PIN material is modulated causing the optical signal absorption properties of the material to vary in accordance with the modulation signal. As a result, the intensity of the optical wave changes in accordance with the applied modulation voltage signals.
Another example of the state of the art wherein DAC and modulator functionalities are merged is given in [6] Y. Ehrlichman et al., J. Lightwave Technol. 29(17), 2545 (2011); see also, [7] http://www.ieee802.org/3/100GNGOPTX/public/nar12/plenary/dama_01_0312_NG100GOPTX.pdf. Here a single (dual-drive) Mach-Zehnder modulator is used and the electrodes in each arm are arranged strictly in a power-of-two length arrangement; that is each electrode length is doubled as compared to the previous positioned electrode. Driving each electrode with equal amplitude binary signals (e.g., +voltage and no voltage signal) generates phase shifts proportional to the electrode lengths in each arm thus converting the digital multi-electrode drive signals to a single analog phase shift per arm. The phase shifted signals in each arm interfere (destructively and/or constructively or both) with each other at the output coupler resulting in a complex-valued analog optical signal. While this modulator is mainly applicable to PAM (Pulse Amplitude Modulation) and in principle is capable of generating a large variety of optical waveforms, it suffers from chirp problems, which is typical of Mach Zehnder modulators. A chirp is an undesired residual phase modulation that can lead to signal distortion. A chirped signal is characterized by an unwanted optical phase modulation accompanying the intentional modulation of amplitude and/or phase. Furthermore, it is known that the bandwidth of an electrode depends on its length. Relatively short length electrodes have generally relatively wider bandwidths. Conversely, relatively longer electrodes have generally relatively narrower bandwidths. Hence, implementing different-length electrodes results in different bandwidths for the individual digital electronic bits to be converted to the analog optical domain, which causes non-linear (and thus difficult to equalize) signal integrity problems.